The Carboniferous was a period in Earth's history, lasting from 359.2 to 299 million years ago. Because of the vast amount of forests growing across the globe during this period, the atmosphere had much higher oxygen levels than the present, allowing gigantic species of insects to evolve and thrive.
Visits and incursions
An Anomaly to the Carboniferous opened in the London Underground disused tunnels in the present, and a colony of Carboniferous Arachnids and one Arthropleura came through. The Arthropleura was killed via electrocution, and the Arachnids were driven back through the Anomaly using light from torches. Helen Cutter apparently waited in the Carboniferous on the other side of the Anomaly for Nick to come after her, but Nick ultimately decided against going through in search of Helen.
An Arthropleura was captured for use in Oliver Leek'screature army, likely from the London Underground Anomaly. It was presumably killed when the creatures were locked in the Creature Prison's cage room and destroyed each other.
The Carboniferous period lasted from approximately 359 to 299 million years ago. During the Carboniferous, the atmosphere's oxygen content was at 35%, which was 14% higher than today's oxygen levels and was the highest level the oxygen content ever reached. The land was covered with vast swamps and rainforests, and land-based life had become well-established. Amphibians were the dominant terrestrial vertebrates, and arthropods, some of which grew to very large sizes, were also highly common.